Fabrication of the Pattern of Copper Nanowires with Adjustable

Nov 24, 2009 - The pattern can be controlled by varying the frequency of the applied voltage. ... the parallel copper wires pattern by the KHSO5/H2SO4...
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J. Phys. Chem. C 2009, 113, 21303–21307

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Fabrication of the Pattern of Copper Nanowires with Adjustable Density on Oxidized Si Substrate Chang Liu, Binbin Yao, Shuangming Wang, Huifang Tian, and Mingzhe Zhang* State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun, P. R. China ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009

We report a method of combining of ultrathin liquid layer electrodeposition with chemical microetching, to fabricate the pattern of ultralong (L > 100 µm) copper nanowires with adjustable density on a two-dimensional oxidized Si substrate. In the electrodeposition process, we employ a periodic variation voltage in an ultrathin layer of concentrated CuSO4 electrolyte. The periodic nano/microstructures with alternate homogeneous membrane and ridgelike wire stripes are constructed by the electrochemical deposition. The pattern can be controlled by varying the frequency of the applied voltage. The homogeneous membrane stripes can be etched out to form the parallel copper wires pattern by the KHSO5/H2SO4 mixture solution. The KHSO5/H2SO4 etchant shows a good stability in microetching. The homogeneous membrane makes it easier to control the removal process using the chemical microetching. The density of wires can be adjusted by changing the frequency of applied voltage, and the large area pattern can be obtained. A suitable explanation has been given for the growth mechanism of periodic structures, and the mechanism of etching. This method is simple and controllable and can be applied to fabricate other metal patterns having potential applications in microelectronics and optoelectronics. 1. Introduction The microscopic parallel wires patterns are attractive for their potential applications as interconnects in future generations of nanometer-scale electronics.1-5 Copper nanowires are already extensively used as interconnects deep within some types of integrated circuits, especially microprocessors.6 The ability to fabricate microscopic structures is important for creating functional nanoscale devices. However, to synthesize ultralong nanowires (L > 100 µm) and to have control over the location, two barriers must be overcome.7 Electrodeposition is one of the methods used to produce such regular microstructures. It has been used to produce materials with nano/microstructures that cannot be built with other traditional techniques, from well-ordered structures to many unexpected forms.8-13 These nano/microstructure patterns can be deposited on a substrate, either with or without template assistance or with induced additives.9-15 In the thin layer electrochemical deposition, a variety of growth morphologies have been observed, depending on the quasi-two-dimensional experimental conditions,16-18 which are mainly governed by the Laplacian fields.19,20 We discovered a new approach to potential-induce copper periodic micro/nanostructures with alternate membrane and wire stripes.21 After etching out the periodic membrane stripes, make it possible to prepare the ultralong wires pattern. However, the uneven membrane thickness we fabricated before has an impact on the removal. Recently, by controlling the electrodeposition conditions precisely, we fabricate periodic nano/microstructures with alternate homogeneous membrane and ridgelike wire stripes. The homogeneous membrane thickness makes it easier to control the microetching process, which lays the foundation for the fabrication of ultralong wires. * Corresponding author. Phone: 86-0431-85166089. Fax: 86-043185166089. E-mail: [email protected].

Figure 1. (a-d) Schematic diagrams showing the procedure of fabricating the Cu wires on the substrate. (e) The setup for electrodeposition. (f) Schematic diagram showing the growth process of deposits.

In this paper, we fabricate periodic nano/microstructures with alternate homogeneous membrane and ridgelike wire stripes by controlling the experimental conditions for electrodeposition precisely. Then, etching out the homogeneous membrane stripes by chemical microetching, a large area regular ultralong nanowires pattern with adjustable density is obtained. A suitable explanation has been given for the growth mechanism of periodic structures, and the mechanism of etching. 2. Experimental Section The experimental steps are shown in Figure 1 which includes the following: (a) Silicon substrates (WaterNet Co., Type N, 0.5 mm thick, orientation [100], resistivity 2-5 Ωcm) were cut into pieces about 20 × 20 mm2 before they were successively cleaned ultrasonically in acetone, chloroform, ethanol, and Millipore water for 10 min, rinsed with Millipore water, and

10.1021/jp9064278  2009 American Chemical Society Published on Web 11/24/2009

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Figure 2. Relationship between the periodic structures with wire and membrane stripes and the applied voltage of semisine wave across the electrodes under experimental conditions. (a) The voltage varies from 0.7 to 1.0 V. (b) SEM image showing the copper grown on the silicon substrate, the growth direction, and three distinctive regions: (I), (II), and (III). The inset in the upper right corner is the AFM image, and the lower right corner is the AFM section image, which shows the average height of the wires is 100 nm. The inset in the left is the TEM image.

dried with high-purity nitrogen. (b) They were treated with an oxygen plasma for 3 min at 300 W (Templa System 100-E plasma system) for oxidation and rinsed again with deionized water. The oxide layer was about 50 nm thick. (c) Applying semisine wave voltage to electrodeposit periodic nano/microstructures with alternate membrane and wire stripes whose densities depend on the frequency of the applied voltage. (d) Finally, etching out the membrane stripes to form the parallel ultralong copper wires pattern. The setup for electrodeposition is shown schematically in Figure 1e. The cathode and anode were made of 40 µm thick copper foil (99.9%) and separated from each other by 8 mm on an oxidized Si substrate. Then, drip a drop of electrolyte between two electrodes. The electrolyte was prepared by analytical reagent CuSO4 (Fluka) and Millipore water. The concentration of the CuSO4 electrolyte is 0.5 M, and the pH value is 4.432. Finally, a piece of glass was covered on these. The setup was placed in a closed growth chamber, the temperature of which was controlled by a Peltier element and a temperature selector (the circulators with programmable or digital controller). When the temperature was declined to -2.35 °C, the electrolyte was solidified. The solute of CuSO4 was partially expelled from the ice in the solidification process due to the partitioning effect.22 Eventually, an ultrathin layer of concentrated CuSO4 electrolyte was formed between the ice of the electrolyte and the lower silicon substrate. A constant voltage was applied across the electrodes, causing the electrodeposition to occur, shown in Figure 1f.

The deposits were then placed in KHSO5/H2SO4 microetchant solution which contains 0.05 wt % oxone and 1 wt % H2SO4 at 40 °C. Part of the sample was removed after 180 s and then washed with ultrapure water repeatedly and dried. 3. Results The applied voltage was a half-sine wave voltage with a function of |V ) 0.3 sin(ωt)| (where ω ) 2πf, shown in Figure 2a), and a periodic nano/microstructure with alternate membrane and wire stripes (shown in Figure 2b) was obtained. When the frequencies of the applied voltage were 0.3, 0.6, and 0.9 Hz, the corresponding electrodepositon areas were I, II, and III in Figure 2b, respectively, indicating that cycle width decreases with increasing frequency. Every cycle was composed of membrane and wire stripes. The membrane stripes were homogeneous, and the ridgelike wire stripes were symmetrical; the average height of the wires is about 100 nm (as shown in the AFM image of Figure 2b). To explain the growing process more intuitively, the schematic diagram (Figure 1f) of growing direction was given. In the schematic diagram, the overall growing direction of depositions is from the cathode to the anode; they grow perpendicular to the cathode. The direction of Cu nanowires is inconsistent with the growing direction of deposits; they are parallel to the cathode. A vivid metaphor for this is that the electrodeposits flock to the anode, just like the waves, and a Cu nanowire is a wave crest. To investigate the

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Figure 3. The relationship between the periodic structure and the applied voltage across the electrodes where the frequency of the semisine wave was 0.6 Hz: (a) the voltage varies from 0.7 to 1.4 V; (b) the voltage varies from 0.7 to 1.0 V; (c) the voltage varies from 0.7 to 0.8 V.

Figure 4. Scanning electron micrograph showing the copper wires pattern on a silicon oxide surface. The copper wires are straight, and the space between wires changes from wide to narrow. The inset in the upper right corner is the height profile and the topographical AFM image of a regular pattern of copper wires on a substrate. The inset in the left is the TEM image.

real-time growth process further, we found that, though ridgelike wires were narrow, the growth was significantly slow relative to that of membrane stripes. It accounted for about 1/3 of the growth time in one period. The inset in the left of Figure 2b is a TEM image of the deposits, where the high density region in the middle corresponds to the wire stripe and sparse density regions on both sides correspond to the membrane stripes. The electron diffraction image of a portion of the wire and membrane shows polycrystalline rings corresponding to Cu(111), Cu(200), Cu(220), and Cu(311), which confirms that the deposits are composed of copper nanoparticles. To a large extent, the morphology of deposits depends on the amplitude value of given voltage. When the amplitude value of the applied voltage exceeds 0.6 V, the deposition pattern shows an analogous branching pattern in the plane membrane stripes region along with distinct ridgelike wires (Figure 3a), which was reported before.21 When the amplitude value is less than 0.1 V, the distinction between membrane region and ridgelike wire region becomes obscure (Figure 3c). Only when the amplitude value is between 0.1 and 0.6 V, the homogeneous plane membrane and ridgelike wires can be obtained (in Figure 3b).

After microetching, the homogeneous membrane stripes were removed and left with wire stripes which formed large area ultralong (L > 100 µm) copper wires, as shown in the SEM image of Figure 4. The inset in the upper right corner is an AFM image of regular copper wires. The average height of the wires after etching is 70 nm shown in the AFM section image, in which the tiny undulation between two wires in deposits profile image due to the disorder of particles became smooth, indicating that the membrane stripes were etched completely. The inset in the left of Figure 4 is the TEM image and electron diffraction image of the sample after etching. The TEM evidence indicates that the chemical microetching changed the morphology of the samples but not the composition. 4. Discussion According to the Nernst equation, the deposition potential of copper ions can be obtained as 0.34 vs SHE. The given lowest deposition voltage in our experiment reaches the requirement of the copper deposition potential. The periodic nano/microstructures with alternate membrane and wire stripes is mainly attributed to two reasons: (1) the variation of copper ion

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Figure 5. Schematic diagram showing the distribution of electron and copper ion concentration in time and comparing it with the spatial distribution of the electrodeposits. (I) The distribution of electron concentration on the growth front. (II) The distribution of copper ion concentration near the growth front. The b-d area corresponds to a unit growing a period of deposits with wire and membrane stripes, the b-c area corresponds to membrane stripes, and the c-d area corresponds to wire stripes.

concentration in the solution near the growth front and (2) the variation of electron concentration on the growth front. In the electrodeposition process, when the electrons on the growth front are superfluous, the copper ions near the growth front are relatively few compared with the abundant supply of electrons. To get ions in the solution, the deposit protrudes quickly into the electrolyte, leading to combination of electrons and ions, reduction reactions, nucleation, and growth. This eventually leads to the formation of the plane membrane stripes region with sparse particle density. When the copper ions near the growth front are superfluous relative to electrons, part of the ions are deoxidized and the rest of the surplus copper ions accumulate at the front of the deposits. The reduction reaction can occur directly at the growth front, and the deposits need not protrude into the electrolyte to capture ions. Thus, the momentum of growing toward the cathode is inhibited, leading to extremely slow growth and eventually forming ridgelike wire stripes with much larger particle density than that of membrane stripes.23 In the process of quasi-two-dimensional ultrathin layer deposition, both ion diffusion and ion convection are restricted, and the electric migration is dominant.24 When the applied voltage is changed, the electron concentration on the growth front changes synchronously. At the same time, copper ions suffer the function of resistance; the speed moving toward the cathode is finite. When the electron concentration on the growth front reaches the maximum, the copper ion concentration near the growth front still lags behind, showing a hysteresis.21 However, both concentration curves are half-sine waves with equal period (shown in Figure 5). Curve I shows the concentration variation of electrons on the growth front; curve II shows the concentration variation of copper ions near the growth front. From the AFM image in Figure 2, every ridgelike wire stripe is symmetrical. The symmetry of the wire stripe requires the deposition to meet two conditions: the electron concentration is symmetrical in the region of growing wire stripes and so as the copper ion concentration. Only when electron concentration reaches the minimum with applied voltage, and at the same time the copper ion concentration near the growth front reaches the maximum, can the amount of reduction reactions be symmetrical, and the morphology of the deposits be symmetrical, as shown in Figure 5. At point a, electron concentration reaches the minimum with applied voltage. At the same time point a′, copper ion concentration near the growth front reaches maximum. Thus, the variation of copper ion concentration near the growth front (curve II) should lag behind the variation of electron concentration on the growth front (curve I) by half a

Liu et al. period. The growth time of wire stripe accounts for about 1/3 of a whole growth period, which is consistent with the realtime growth process. The distance between c and d (which are intersection points of curve I and curve II) is 1/3 of one growth period (points b-d). The periodic nano/microstructures with alternate membrane stripes and wire stripes are fine-structure composed of nanoparticles and the figures (the AFM and TEM images in Figure 2) suggest that the height and density of the wire and membrane are inconsistent. The density of wire stripes is bigger than that of membrane stripes and the membrane stripes thickness is low relative to that of wire stripes, which provides favorable conditions for the preparation of the nanowires pattern. It makes the membrane stripes tend to be etched and left with wire stripes to form a large area ultralong Cu wires pattern. At the same time, the uniformity of membrane stripe is beneficial to be removed simultaneously, which makes it easier to control the etching time. Because of the fine-structure of deposits, the stability of the microetchant is strictly required Since the etch rate directly determines the time required to get the desired etch depth, it is the most important parameter in the stability of microetching. In our experiment, the KHSO5/ H2SO4 mixture with the stable etch rate is employed. The reasons for the stability of etch rate are embodied in the etching mechanism, which can be described as follows. The main etching reaction on the copper surface is

H2SO4 + KHSO5 + Cu f CuSO4 + KHSO4 + H2O (1) and the microetching process is also accompanied by another reaction:

Cu + Cu2+ f 2Cu+

(2)

In general, the etch rate is mainly determined by the sample composition, etching temperature, and etchant composition. Because the composition of the sample is fixed, and the etching temperature is fixed at 40 °C, only the etchant composition will be considered. Here, we discuss the effect of the concentrations of KHSO5, H2SO4, and Cu2+ on etch rate, respectively. Clearly, the higher the concentration of KHSO5, the faster the etch rate. For better etching effect, we’d like to extend the etching time and prepare the microetchant with a dilute concentration of 0.05 wt % oxone. In the early etching, the high concentration of H2SO4 leads to the high concentration of HSO4- in the product, which inhibits the positive reaction in eq 1, so the high etch rate is limited. As the etching proceeded, the concentration of H2SO4 got smaller, leading to the decrease of etch rate. However, the low concentration of H2SO4 leads to the low concentration of HSO4-, which accelerates the positive reaction in eq 1 and inhibits the decrease of etch rate. Anyway, the existence of HSO4- makes a cushioning effect on the change of etch rate. Thus, the etch rate is relatively stable. The concentration of Cu2+ also affects the etch rate. In general, the etch rate should decrease with the increased concentration of product Cu2+. Actually, higher concentration of Cu2+ corresponds to a faster etch rate, which is because of the existence of reaction 2. With the increased concentration of Cu2+ generated by eq 1, the oxidation-reduction potential of Cu2+/Cu+ also increases gradually, leading to the increase of the etch rate. However, when the Cu2+ concentration reaches a certain value, its effect on reaction eqs 1 and 2 tends to balance and the etch rate tends to stabilize.

Fabrication of the Pattern of Copper Nanowires Because of the stable etch rate, the more controllable degree of etching can be observed by optical microscope. It is found that, at 180 s, the membrane stripes just have been etched completely. Comparing images before and after etching (the AFM images in Figure 2 and 4), it can be found that the vertical distance between the wire surface and membrane surface changes from 26 to 77 nm, indicating that the etching depths of different parts are not the same. This is can be explained that on the same of other conditions, the etching qualities are equal for different parts. However, the density of wires is larger than that of membranes, so the etching depth of wire stripes is less than that of membrane stripes, leading to the increase of the vertical distance, which confirms that the structures we deposited provide favorable conditions for the preparation of nanowires pattern. 5. Conclusions To conclude, we report a simple and controllable method of combining ultrathin liquid layer electrodeposition with chemical microetching, to fabricate the ultralong copper wires pattern with adjustable density on oxidized Si substrate. The density of wires can be adjusted by changing the frequency of applied voltage, and the large area desired pattern can be obtained. This is an interesting approach to fabricate Cu nanowires, and we need to emphasize that this technique is not only useful for making the copper wires pattern but also can be applied to fabricate other metal and semiconductor nanowire patterns having potential applications in microelectronics and optoelectronics. Acknowledgment. This work was funded by the National Science Foundation of China, Nos. 50672029, 90923032, and 20873052. It was also supported by the Ministry of Science and Technology of China, No. 2005CB724404. References and Notes (1) Chai, J. N.; Wang, D.; Fan, X. N.; Buriak, J. M. Nat. Nanotechnol. 2007, 2, 500.

J. Phys. Chem. C, Vol. 113, No. 51, 2009 21307 (2) Ieong, M.; Doris, B.; Kedzierski, J.; Rim, K.; Yang, M. Science 2004, 306, 2057. (3) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem.; Int. Ed. 2004, 43, 30. (4) Huang, X. J.; OMahony, A. M.; Compton, R. G. Small 2009, 5, 776. (5) Walter, E. C.; Zach, M. P.; Favier, F. R.; Murray, B. J.; Inazu, K.; Hemminger, J. C.; Penner, R. M. ChemPhysChem 2003, 4, 131. (6) Murarka, S. P.; Verner, I. V.; Gutmann, R. J. CoppersFundamental Mechanisms for Microelectronics Applications; Wiley: New York, 2000. (7) Xiang, C. G.; Yang, Y. G.; Penner, R. M. Chem. Commun. 2009, 859. (8) Shao, W. B.; Zangari, G. J. Phys. Chem. C 2009, 113, 10097. (9) Zhang, M. Z.; Zhuo, G. H.; Zong, Z. C.; Cheng, H. Y.; He, Z.; Yang, C. M.; Li, D. M.; Zou, G. T. Appl. Phys. Lett. 2006, 88, 203106. (10) Yu, R.; Ren, T.; Sun, K. J.; Feng, Z. C.; Li, G. N.; Li, C. J. Phys. Chem. C 2009, 113, 10833. (11) Zhang, M. Z.; Zhuo, G. H.; Zong, Z. C.; Cheng, H. Y.; He, Z.; Yang, C. M.; Zou, G. T. Small 2006, 2, 727. (12) Belding, S. R.; Dickinson, E. J. F.; Compton, R. G. J. Phys. Chem. C 2009, 113, 11149. (13) Zhang, M. Z.; Lenhert, S.; Wang, M.; Liu, N.; Chi, L. F.; Fuchs, H.; Ming, N. B. AdV. Mater. 2004, 16, 409. (14) Borissov, D.; Uppenkamp, S. I.; Rohwerder, M. J. Phys. Chem. C 2009, 113, 3133. (15) Leger, C.; Elezgaray, J.; Argoul, F. Phys. ReV. E 2000, 61, 5452. (16) Kuhn, A.; Argoul, F.; Muzy, J. F.; Arneodo, A. Phys. ReV. Lett. 1994, 73, 2998. (17) Fleury, V.; Chazalviel, J. N.; Rosso, M. Phys. ReV. Lett. 1992, 68, 2492. (18) Chazalviel, J. N.; Rosso, M.; Chassaing, E.; Fleury, V. J. Electroanal. Chem. 1996, 407, 61. (19) Witten, T. A.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400. (20) Grier, D. G.; Ben-Jacob, E.; Clarke, R.; Sander, L. M. Phys. ReV. Lett. 1986, 56, 1264. (21) Zong, Z. C.; Yu, H.; Nui, L. P.; Zhang, M. Z.; Wang, C.; Li, W.; Men, Y. F.; Yao, B. B; Zou, G. T. Nanotechnology 2008, 19, 315302. (22) Markov, I. V. Crystal Growth for Beginners. Fundamentals of Nucleation, Crystal Growth and Epitaxy; World Scientific: Singapore, 1995. (23) Oberholtzer, F.; Barkey, D.; Wu, Q. Phys. ReV. E 1998, 57, 6955. (24) Wang, M.; Zhong, S.; Yin, X. B.; Zhu, J. M.; Peng, R. W.; Wang, Y.; Zhang, K. Q.; Min, N. B. Phys. ReV. Lett. 2001, 86, 3827.

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